This invention generally relates to spatial light modulators and in particular relates to a polarization-based modulator comprising an array of micro-mechanical assemblies.
Spatial light modulators have been adapted for use in a range of imaging applications, particularly in projection and printing apparatus. In operation, a spatial light modulator array provides a pattern of individual light modulators, each modulator typically corresponding to a pixel for representing a two-dimensional image. Light can be modulated by modifying the incident light according to selective absorption, reflection, polarization state change, beam steering, diffraction, wavelength separation, and coherence modification. Typically, the interaction of the light and modulator is enabled using electro-optic or acousto-optic materials, or a micro-mechanical structure, patterned with a series of addressing electrodes.
In particular, the liquid crystal display (LCD) is a widely used type of spatial light modulator, which operates by the modulation of the polarization state of incident light. LCDs are commonly used in laptop computer displays, pagers, and game displays, as well as in projection and printing systems. LCD spatial light modulators are available in a range of types and may use any of a number of underlying technologies, including devices using nematic, twisted nematic, cholesteric, smectic, and vertically aligned liquid crystal molecules. LCDs are described in numerous patents, including for example, U.S. Pat. No. 4,688,897 (Grinberg et al.); U.S. Pat. No. 5,039,185 (Uchida et al.); U.S. Pat. No. 5,652,667 (Kuragane); and U.S. Pat. No. 5,847,789 (Nakamura et al.). LCDs are also available in a wide range of sizes, from devices suited to micro-displays to devices used for direct view laptops. LCD performance characteristics, such as response time, angular acceptance, contrast, and control voltages, vary depending on the device.
Spatial light modulators that alter the polarization state of incident light have also been constructed using lead lanthanum zirconium titanate (PLZT), as described in U.S. Pat. No. 4,707,081 (Mir), U.S. Pat. No. 4,887,104 (Kitano et al.), and U.S. Pat. No. 5,402,154 (Shibaguchi et al.). While PLZT devices are robust relative to optical damage thresholds, these devices typically have modest modulation speeds (kHz range), require high drive voltages, and have electro-optic response curves with significant hysteresis.
LCD and PLZT devices are suitable for many applications, but have a number of inherent disadvantages, including relatively slow response times (typically a few ms) and significant optical response variations relative to the angle of incidence. Most LCD modulators are unable to provide both high modulation contrast and fast modulation speeds simultaneously. Modulation contrast not only varies with angle and wavelength, but can also be degraded by thermally induced stress birefringence when exposed to the large light loads common to projection applications. In demanding applications using LCDs, the systems are often enhanced through the use of carefully designed polarization compensators (for example see U.S. Pat. No. 4,701,028 (Clerc et al.) and U.S. Pat. No. 6,081,312 (Aminaka et al.), which boost contrast, but at the cost of additional optics to the system.
One approach to providing spatial light modulators with improved response time is to adapt micro-mechanical devices to this task. The digital micro-mirror device (DMD) from Texas Instruments, Dallas, Tex., as disclosed in U.S. Pat. No. 5,061,049 (Hornbeck), is one such device, which modulates by beam steering the incident light relative to the imaging optics. Micro-mechanical gratings, including the grating light valve (GLV), disclosed in U.S. Pat. No. 5,311,360 (Bloom), and the conformal grating modulator, disclosed in U.S. Pat. No. 6,307,663 (Kowarz), have been successfully developed. These gratings impart a phase pattern to the incident light, causing it to diffract when modulated. Both the micro-mirror and the grating modulators require the use of a Schlieren type optical system, with blocking apertures or angular separation, to distinguish between the modulated and un-modulated light. Alternately, a spatial light modulator with rolling micro-mechanical shutters is described in U.S. Pat. No. 5,233,459 (Bozler et al), which either blocks or transmits the incident light, according to the control signals. As compared to the electro-optical or acousto-optical devices, the micro-mechanical modulators typically provide a more uniform response, both within a device (from pixel to pixel) and relative to the properties of the incident light (angle of incidence, wavelength, etc.). The micro-mechanical optical modulators also typically provide faster response times (On to Off, and visa-versa) than do many of the electro-optical devices. While these devices have provided some improvements in performance, there is room for improvement. For example, DMD devices are capable of achieving higher speeds, but are presently limited in achieving high resolution, and limit the input light to a modest angular beam width ( less than 10xc2x0 or less than F/3.0). By comparison, the GLV and related devices are generally limited to one dimensional structures, due to optical fill factor issues between adjacent rows.
While micro-mechanics have been applied to light modulation using beam steering, diffraction, and beam blocking mechanisms, there are further opportunities to bring the advantages of micro-mechanical (MEMS) structures to the area of optical modulation. In particular, an improved polarization modulator could be designed, with potentially faster response times and more uniform angular and wavelength responses as compared to some of the conventional electro-optical devices.
Micro-mechanical structures, which might be adaptable to the construction of a micro-mechanical polarization modulator, have been described, including motors, rotors, and mini-turbines. Exemplary structures and manufacturing processes for micro-motors are discussed in numerous prior art patents, including U.S. Pat. No. 5,252,881 (Muller et al.), U.S. Pat. No. 5,710,466 and U.S. Pat. No. 5,909,069 (both to Allen et al.), and U.S. Pat. No. 5,705,318 (Mehregany et al.). Micro-motors have been fabricated and tested on a scale as small as 60-100 xcexcm diameter, which is of a size appropriate for building a pixilated spatial light modulator, although smaller motor diameters could be useful. U.S. Pat. No. 5,459,602 (Sampsell) and U.S. Pat. No. 5,552,925 (Worley) describe micro-motors that are adapted with revolving blade shutters. Alternately, U.S. Pat. No. 6,029,337 (Mehregany et al.) describes a micro-motor structured to facilitate the creation of a variety of devices, including a micro-polygon scanner and micro-grating optical scanner. In particular, FIG. 4 of U.S. Pat. No. 6,029,337 illustrates the concept of a rotating diffraction grating (long pitch (p greater than  greater than xcex)), mounted to a micro-motor, and used in an optical scanner. These devices, operating at rotational speeds up to 50,000 rpm (1.2 msec/rev.), can be used in optical systems for a variety of applications, including bar code scanners and laser printers.
However, U.S. Pat. No. 6,029,337 neither describes the design and construction of a micro-mechanical polarization spatial light modulator, nor anticipates the potential advantages of such a device and its application within a modulation optical system. In particular, such a device is necessarily fabricated with a surface structure that alters the polarization state of the incident light in accordance with it rotational position. Traditionally, optical polarizers have been constructed with bulk materials, such as crystal calcite, or as the Polaroid type dye sheets with stretched polymers, or as optical thin films within glass substrates (U.S. Pat. No. 2,403,731 (MacNielle)), or finally as aligned metallic needles embedded in a glass medium (U.S. Pat. No. 5,281,562 (Araujo et al.) and U.S Pat. No. 5,517,356 (Araujo et al.). Although these various types of polarizers are valuable in their own right, they do not lend themselves to integration with a micro-mechanical structure. In particular, these types of polarizers tend to be both large in scale (millimeters and centimeters in extent) and use fabrication processes not conducive to the miniaturization. Furthermore, even if these polarizer types were fabricated on the sub-millimeter scale, they are not readily attached or integrated onto a micro-mechanical device.
Polarizers can however be manufactured with processes that lend themselves to modern manufacturing techniques for miniaturization and patterning. Furthermore, such polarizers can be manufactured for the visible wavelength range, rather than the infra-red wavelength range, where such form-birefringent and form-dichroic structures were previously limited. Form-birefringent, all dielectric, sub-wavelength structures have been developed for use as polarization sensitive mirrors, polarizing beansplitters, and waveplates. In such structures, a sub-wavelength grating structure is formed in a dielectric material, with various parameters, including the pattern, groove period, groove profile, and groove depth, determining the performance of the device. As an example, the paper xe2x80x9cDesign considerations of form-birefringent micro-structuresxe2x80x9d, I. Richter et al., Applied Optics, Vol. 34, No. 14, pp. 2421-2429, May 1995, discusses many of the design compromises and issues in the design of such structures.
Optical polarizers with sub-wavelength structures can also be designed and fabricated with mixed metal-dielectric structures. In particular, U.S. Pat. No. 6,122,103 (Perkins et al) and U.S. Pat. No. 6,243,199 (Hansen et al) describe visible wavelength wire grid polarizers and polarization beamsplitters fabricated from sub-wavelength metallic wire deposited on a glass substrate. As compared to the all dielectric devices, the wire grid devices typically provide greater differences in response (higher contrast) for the transmitted polarization vs. the reflected polarization. However, both the dielectric-form-birefringent polarizers and the wire grid polarizers generally provide polarization responses that are generally uniform over extended wavelength ranges and large ranges of incident angles.
In general, the use of dielectric-form-birefringent polarizers and the wire grid polarizers has been applied to static optical devices, such as waveplates, polarizers, and polarization beamsplitters, which reside in a pre-determined position within an optical system. Spatial light modulators, such as liquid crystal displays, may also exist within these systems, and provide the actual data input modulation. However, the operation of such systems is then typically limited by the polarization response of the liquid crystal displays, and the full polarization response of the sub-wavelength structured polarizer is under utilized. If, on the other hand, the use of sub-wavelength structure polarizers were applied to the construction of the modulator itself, the overall response of the optical systems could be improved. In particular, there has been no attempt to adapt sub-wavelength structured optical retarders and polarizers in the construction of spatial light modulators using micro motors.
Thus it can be seen that there is an opportunity for a spatial light modulator that operates by polarization modulation, employing sub-wavelength structured optical polarizers controlled by micro-mechanical actuators, which are preferably micro-mechanical micro-motors.
It is an object of the present invention to provide a spatial light modulator for modulating the polarization state of an incident beam.
Briefly, according to one aspect of the present invention a micro-mechanical spatial light modulator for modulating a polarization state of an incident beam comprises a plurality of rotatable elements. Each rotatable element comprises a plurality of structures. The structures are spaced apart within sub-wavelength distances relative to the wavelength of the incident beam. Each of the plurality of structures exhibits an interaction with the polarization state of the incident beam. A micro-motor is coupled to each of the rotatable elements. The micro-motor is capable of controllably positioning the rotatable element to at least two positions. Each position has a corresponding polarization state. A substrate structure supports each of the plurality of rotatable elements and houses each micro-motor.
A feature of the present invention is that it provides an array of micro-mechanical structures comprising sub-wavelength optical polarization modulators arranged in a two-dimensional array, wherein each modulator is independently actuated by a micro-mechanical micro-motor.
It is an advantage of the present invention that it provides a spatial light modulator that is capable of providing uniform angular response with fast response times.